1. Field of the Invention
The present invention relates to an intermediate coupling unit that couples two image forming apparatuses, each of which can singly form images, for making high speed printing, printing with an additional application of special toner, improving image quality, etc., and to an image forming apparatus adaptable to the intermediate coupling unit.
2. Description of the Related Art
There have conventionally been color image forming apparatuses that form an image by forming a toner image on an image support according to an electrophotographic manner using toner, transferring the toner image onto a sheet of paper, and fixing the transferred toner image on the sheet of paper by a pressure and heat using a fixing unit.
Such electrophotographic image forming apparatuses are required to have a higher image forming speed year by year. A measure that has been employed to realize the higher image forming speed is to increase a rotation linear velocity of a paper conveying mechanism, an image forming unit, etc.
However, if the rotation linear velocity is simply increased, it will become necessary to raise the temperature of a heat generating roller in order to maintain the fixing function of a fixing unit, or to newly install a fixing roller having a large diameter in order to maintain the fixing nip time.
These two measures have their own problems: raising the fixing temperature will result in a large power consumption, and increasing the diameter of the fixing roller will make it hard for a toner-fixed sheet of paper to be detached from the fixing roller.
As a solution to these problems, Unexamined Japanese Patent Application KOKAI Publication No. 2007-163809 proposes an image forming apparatus that includes two image forming units and two fixing units. According to this solution, the print processing capacity is doubled even at a normal speed because there are two image forming units. Furthermore, since the two image forming units that print at the normal speed each have a fixing unit, there is no need of raising the temperature of the heat generating roller or increasing the diameter of the fixing roller.
Further, recently, from the cost and resource saving concerns, double-side printing has been promoted in image forming apparatuses such as copying machines, facsimiles, etc. It has conventionally been general to provide an image forming apparatus with a sheet overturning mechanism to make the image forming apparatus capable of double-side printing. However, since a sheet overturning mechanism necessarily entails a switchback behavior, the operation slows down accordingly, which has been a factor that hinders the speeding up of the apparatus on the whole.
As a method for solving such a problem and realizing fast double-side printing, Unexamined Japanese Patent Application KOKAI Publication No. H09-054465 proposes fast double-side printing realized by preparing two printers that have a single-side printing function and setting the printers' transfer units to face each other so that a sheet of paper that is output from the paper discharge port of the first printer may be conveyed to the paper feed port of the second printer.
However, the technique of Unexamined Japanese Patent Application KOKAI Publication No. 2007-163809 is a configuration of a last printing-only machine that includes two image forming units and two fixing units in one housing. At a glance, this machine looks large, expensive, and intricate to handle, and is not suitable for such offices where a normal printing speed is good enough and not much paper is consumed. Hence, this machine is problematic in that it lacks universal applicability, from the standpoint of the seller of the machine.
The technique of Unexamined Japanese Patent Application KOKAI Publication No. H09-054465 inevitably takes a lateral arrangement of the two printers and requires double the space of a single printer for installation, which is likewise not suitable for small offices.